1. Field of the Invention
The present invention relates to optical disks and optical disk apparatuses, and more specifically to an optical disk and an optical disk apparatus in which an address information start mark is detected from a header region.
2. Description of the Related Art
An optical disk has a plurality of sectors constituting tracks arranged spirally or concentrically. Each sector has a recording region for recording user data, and a header region preceding the recording region, in which address information regarding the position of the sector is recorded. Thus, an optical disk apparatus is required to read the address information recorded in the header region before playing back the user data recorded in the recording region of the optical disk.
In order to acquire the address information, the optical disk apparatus needs to first detect an address mark in the header region, indicating the beginning of the address information, which serves to achieve synchronization when the address information is read.
For example, let it be supposed that a logic HIGH level corresponds to a space and a logic LOW level corresponds to a mark. In the following description, the pulse width of a playback clock of the optical disk apparatus will be denoted as “T”. For example, an address mark AM is represented by a first recording pulse sequence 106 consisting of a mark sequence having a pulse width of 6T and a second recording pulse sequence 109 consisting of a space sequence having a pulse width of 9T, as shown in the lower part of FIG. 6A.
In the following description, a “recording pulse sequence” refers to a sequence of pulses obtained by NRZI (Non Return to Zero Inverted) modulating a bit sequence of variable-length codes. For example, a recording pulse sequence shown in the lower part of FIG. 6A is obtained by NRZI modulating a bit sequence of variable-length codes shown in the upper part of FIG. 6A. In the following description, the pulse width of contiguous spaces in a recording pulse sequence will be referred to as a “space length”, and the pulse width of contiguous marks in a recording pulse sequence will be referred to as a “mark length”.
More specifically, the first recording pulse sequence 106 and the second recording pulse sequence 109 are obtained by NRZI modulating the bit sequence shown in the upper part of FIG. 6A, i.e., inverting at each “1” and not inverting at “0”s.
A conventional optical disk apparatus, when detecting an address mark AM, converts the data written (pre-recorded on the optical disk) as shown in the lower part of FIG. 6A back into the format shown in the upper part of FIG. 6A. The detection criterion for detecting the address mark AM has been such that when a portion of a bit sequence is detected as representing a “combination of space length and mark length” corresponding to the address mark AM, the portion of the bit sequence is determined as the address mark AM.
The conventional detection criterion for the conventional optical disk apparatus to detect the address mark AM, however, has had the following shortcomings.
For example, if bit errors occur in the bit sequence shown in the upper part of FIG. 6A at three points A, B, and C as shown in FIG. 6B, the bit sequence shown in the upper part of FIG. 6B includes a false address mark AMO consisting of the same combination of space length and mark length as that of the address mark AM, as shown in the lower part of FIG. 6B. When such a false address mark AMO is present, the optical disk apparatus detects the false address mark AMO before detecting the true address mark AM that is supposed to be detected.
As another example, if bit errors occur in the bit sequence shown in the upper part of FIG. 6A at two points D and E as shown in FIG. 6C, the bit sequence shown in the upper part of FIG. 6C includes a false address mark AMO consisting of the same combination of space length and mark length as that of the address mark AM, as shown in the lower part of FIG. 6C. Similarly to the previous example, when such a false address mark AMO is present, the optical disk apparatus detects the false address mark AMO before detecting the true address mark AM that is supposed to be detected. Thus, the conventional optical disk apparatus sometimes fails to accurately play back user data from the recording region.